The present invention is a method of forming thermoplastic containers and specifically an ornamental household pitcher which utilizes the known molding techniques of injection molding and blow form molding of a preform in its semi-molten state.
The use of an injection molded preform which is then blow-formed has been done before as taught in U.S. Pat. No. 4,704,243 to Nilsson et al, which teaches a complex six stage molding process which takes an initial injection molded cylindrical-shaped blank having a closed bottom and forms it in five forming stages into a thin-walled vessel capable of light pressures as required in the soft drink industry. The opening of the vessel in the Nilsson patent is formed in one of the latter forming stages with a stretching mandrel while applicant's opening is formed in the initial injection molding step and is unaffected by the later blow-forming step which changes the basically cylindrical shape of the vessel to a spheroidal shape while retaining the flat bottom shape and the spout and lip of the top of the pitcher.
To injection mold a hollow part having an undercut is difficult and expensive by reason of the fact that the inner core of the mold must be collapsed about itself to remove the part from the mold creating a very complex and expensive core construction. Blow-forming of a shape having an undercut is relative simple to achieve however, it almost always requires some type of movable mandrel in the blow-forming step as illustrated in the above-mentioned patent to Nilsson et al.
The general concept of utilizing an injection molded cylindrical vessel in a blow-forming mold for final shape is generally taught in the Japanese Patent No. 62-95211. In this patent the open end of the cylindrical blank, also referred to as a "preform" is necked down to form a smaller diameter opening and in the third blow-forming stage a split mold is utilized to form a ring-shaped projection at the mouth of the bottle for its final shape.
The present invention is distinguishable over both of the above-mentioned patents in that the flared lip and pouring spout at the mouth of the vessel along with the bottom are formed initially in the injection molding step or stage without any alteration in the blow-forming step. The preform shaped in the injection molding stage is a cylindrical vessel and an opened top with the lip therearound including a pouring spout. While still the thermoplastic temperature range, the preform is placed into a split blow mold fixture with the previously formed lip and pouring spout held in position by the surfaces of the mold and an elastic plug inserted in the opened end of the vessel. Whereupon an air line passing through the plug applies pressure to the interior of the preform thus causing the cylindrical wall of the vessel to flow outward to the shape of the blow mold. This flow of the cylinder wall takes place without altering the shape of the lip, spout and the bottom of the vessel. The particular blow-formed shape as illustrated in the drawing is substantially spherical with a concave, annular ring just below the lip of the vessel.
After removal from the blow-forming mold and cooled, a handle formed of a similar plastic is attached to the vessel through a solvent weld thus forming the ornamental pitcher of the present invention.
The principal object of the present invention is to provide a method of forming a thermoplastic housewares pitcher with a minimum of forming steps and mold cost.
Another object of the present invention is to provide a method of forming an undercut opened top vessel with a simplified two-stage process.
Another object of the present invention is to provide a molding method whereby an initial preform is blow molded to its final shape while retaining the top and bottom areas of the preform in their initial shape.